OCCURRENCE OF THERMOGENIC GAS IN THE IMMATURE ZONE - IMPLICATIONS FROM THE BAKKEN IN-SOURCE RESERVOIR SYSTEM

The Bakken petroleum system has been the subject of intense research. Nevertheless, results of the present organic geochemical study reveale d new and previously unreported findings. Most interestingly, immature (<0.7% R(0)) Bakken Shales are charged with high concentrations of in digenous gas as revealed by thermovaporization-gas chromatography. Bot h closed and open system pyrolysis experiments elucidated that generat ion of thermogenic gaseous hydrocarbons early on in maturation seems t o be a feasible process in the case of the Bakken. Interestingly, a co nsiderable portion of the immature kerogen macromolecule is made up of diaromatic carotenoid structures which might be related to the unusua l high gas generative capacity. A good correlation of gas yield with o rganic richness (TOC) for the immature Bakken might indicate that the light hydrocarbons are adsorbed to the organic matter and are at the s ame time indigenous. Alternatively to the hypothesis of early gas gene ration, the catagenetic/metagenetic formation of gas and its intraform ational migration into the immature zone has to be considered as well. However, the likelihood of intraformational migration within the over pressured Bakken Formation is rather low.